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When observing with Nod & Shuffle, one needs to define a nod vector and distance. This is the direction and the distance that the telescope will move when changing pointing from the object position to the sky position. In theory there is no restriction on the size and direction of the nod, though in practice there are several factors one should consider:

Basic Concepts

The capability of GMOS on Gemini to perform Nod & Shuffle operations is not one of the observing modes for which the instrument was originally designed and developed. The capability was proposed, developed, implemented and tested as an enhancement to the existing GMOS-N instrument from late 2001 to September 2002 by the GMOS Nod & Shuffle Commissioning Team.


Wavelength calibration of GMOS is obtained using GCAL, the facility calibration unit. Flat-fielding is done with GCAL or on the sky during twilight. Baseline calibrations will be obtained for all observations, both queue and classical.


ITC, Sensitivity and Overheads

The Integration Time Calculator (ITC) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions, filters, and GMOS configurations. The ITC was used to generate tables of sensitivity estimates for imaging and spectroscopy modes. The R831 grating is not included in these example sensitivity estimates but is available for science use and is included in the ITC.


Nod and Shuffle

The GMOS Nod & Shuffle mode applies techniques adopted for the infrared to optical MOS, long-slit, and IFU (Gemini-South only) spectroscopy whereby the sky is sampled with the same pixels used to observe the science target.

The advantages of observing with Nod & Shuffle include:

  • Improved sky subtraction
  • Potential increased density of slits

Disadvantages of observing with Nod & Shuffle include: